Pref-1 (Preadipocyte factor-1) is a widely used preadipocyte marker that we originally cloned and identified. Pref-1 is expressed in preadipocytes, downregulated during adipocyte differentiation, and is absent in adipocytes. Pref-1 is synthesized as a plasma membrane protein with 6 EGF-repeats in the extracellular domain. Pref-1 ectodomain is cleaved by TACE to generate the biologically active soluble Pref-1. Gain- and loss-of function studies in cultured cells, as well as in mice in vivo showed Inhibition of adipogenesis by Pref-1 in autocrine/paracrine manner. Thus, whereas Pref-1ablated mice have higher adipose tissue mass, Pref-1 overexpression in adipose tissue causes partial lipodystrophy with ectopic fat storage. Pref-1 activates MEK/ERK to induce Sox9, which in turn suppresses C/EBPb and C/EBPd, to inhibit adipocyte differentiation. However, the Pref-1 receptor at the plasma membrane that initiates Pref-1 signaling is yet to be identified. By using recently developed diazirine photo-reactive linker combined with robust NHS-ester chemistry in situ, the putative plasma membrane receptor for Pref-1 has recently been identified. The goal of this research is to firmly establish and characterize the Pref-1 receptor which is the most proximal and critical component of Pref-1 signaling and to study the downstream components to inhibit adipocyte differentiation. To accomplish this goal, biochemical, cellular and molecular approaches as well as gain- and loss-of function studies in vitro and in vivo will be taken. In the long run, elucidating mechanisms underlying Pref-1 binding to the Pref-1 receptor and its downstream signaling pathway will help to develop strategies to control adipogenesis.